1. Field of the Invention
The present invention relates to a developing unit and an electro-photographic apparatus such as a printer, a facsimile and a copier for actualizing an image employing colored particles such as toner, and more particularly to a developing unit and an electro-photographic apparatus having a feature in a developing process of forming a toner image on the surface of a photosensitive body.
2. Background Art
Conventionally, the printers of electro-photographic method have a developing process for actualizing an image on the surface of photosensitive body using colored particles, and a transfer process for transferring the actualized image of colored particles onto a recording medium. The colored particles may be a powder called a toner exclusively used for electrophotography.
The photosensitive body is evenly charged over its entire surface, and then partially discharged by radiating light. At this time, a potential contrast with a charged region and a discharged region is formed on the surface of photosensitive body to form an electrostatic latent image.
In the developing process, first of all, the toner particles are charged using a developer. The developer is classified into a two-component type consisting of a mixture powder of the toner and carrier beads as magnetic particles and a one-component type of the toner alone. The developer is received in the developing unit, and agitated.
Specifically, a two-component developer is composed of a mixture of the toner as colored particles of resin powder having a particle diameter of about 10 micrometers, and the carrier of magnetic particles having an average particle diameter of 30 to 100 micrometers such as ferrite, magnetite, or iron powder. A one-component developer contains a magnetic powder in the toner and not using the carrier.
In the two-component type developer, the toner is charged due to the friction with carrier beads. Also, in the developer of one-component type, the toner is charged due to the friction with members.
The developer is conveyed to a developing position opposed to an electrostatic latent image on the surface of photosensitive body by a magnet roller called a developing roller. At this time, a xe2x80x9cmagnetic brushxe2x80x9d in which the developer is arranged like a brush along the magnetic force lines is formed in a region opposed to the photosensitive body.
In the manner, a developing method in which the developer is conveyed to the electrostatic latent image on the photosensitive body by the magnetic roller is called a magnetic brush development.
On one hand, a method for actualizing the electrostatic latent image, called a bias development, is often employed. In the bias development, a bias voltage is applied to the developing roller, and the charged toner particles are separated from the developer on the surface of the developing roller, owing to an electric field generated between a latent image potential on the surface of photosensitive body and the developing roller, and moved to the surface of photosensitive body, thereby performing the image formation.
The charging potential or discharging potential may be employed as the latent image potential, or a potential of an image forming portion of the photosensitive body. Generally, the method of using the charging potential as the latent image potential is called a normal development, or the method of using the discharging potential is called a reversing development.
One of the charging potential and the discharging potential that is not used as the latent image potential is called a background potential. The bias voltage of the developing roller is set at a certain value between the charging potential and the discharging potential, and a difference from the latent image potential is called a developing potential difference.
Similarly, a difference from the background potential is called a background potential difference. Usually, the developing potential difference for governing a development performance itself is set to be greater than the background potential difference. With greater developing potential difference, the formed electric field (developing electric field) is increased, so that the development performance (toner developing amount) is enhanced.
Also, increasing the rotating speeds of the developing roller and the photosensitive body, reducing the distance, and decreasing the electrical resistance of the developer have effect on strengthening the developing electric field, making it possible to increase the toner developing amount.
A developing method of using the magnetic brush development to convey the developer, and the bias development to actualize the electrostatic latent image (hereinafter referred to as a xe2x80x9cmagnetic brush bias developmentxe2x80x9d) is widely employed.
A relative moving direction of the developing roller and the photosensitive body may be the same or opposite. Also, one developing unit may employ a plurality of developing rollers. In some developing unites, the plurality of developing rollers are rotated in the same direction, but in other developing unites, they are rotated in different directions.
In this case, a developing unit is well-known in which the adjacent developing rollers are rotated in different rotations from the opposite position of developing rollers toward the photosensitive body, and the developer is branched and conveyed like a fountain from the opposite position of developing rollers to the photosensitive body (hereinafter referred to as a xe2x80x9cfountain-type developing unitxe2x80x9d).
The magnetic brush bias development has a problem that the image edge is difficult to develop in the rotation direction of the developing roller. This problem occurs due to a mechanical factor that the magnetic brush rubs against the surface of photosensitive body, and because the potential of the photosensitive body contacted by the magnetic brush is drastically changed from the background potential of non-image part to the developing potential of image part to change the electrical characteristics of the developer.
Thus, using the fountain-type developing unit, two developing rollers rotated in different directions compensate each other, thereby solving this problem.
The developing roller includes a core having the magnetic poles and a metallic sleeve containing this core, in which the developer is conveyed by rotating the sleeve with the core fixed (hereinafter referred to as a xe2x80x9csleeve rotating methodxe2x80x9d) or rotating both the core and the sleeve (hereinafter referred to as a xe2x80x9ccore rotating methodxe2x80x9d).
In addition, it is called a sleeve rotation developing roll in which the magnetic roll inside the developing roll is fixed. It is called a double rotation developing roll in which the internal magnet roll is also rotated.
The core rotation method is more effective in agitating the developer in a developing area opposed to the photosensitive body than the sleeve rotation method, and has a greater toner developing amount. Therefore, even if the rotating speed of the sleeve is decreased, a sufficient toner developing amount can be assured to reduce the scraped toner image on the photosensitive body caused by mechanical sliding of the developing roller itself, thereby improving the image quality.
The core rotating developing unit was described in JP-A-58-142358.
Further, a developing method using two double rotation developing rolls was described in JP-A-58-142358. As its contents, a first double rotation developing roll rotates the sleeve and the magnet roll in opposite directions, or rotates the sleeve alone, while a second double rotation developing roll rotates the sleeve and the magnet roll in the same direction.
When the fountain-type developing unit having the feature that no defective image edge is caused due to the electrical factor employs at least one of the developing rollers based on the core rotating method (hereinafter referred to as a xe2x80x9ccore rotating fountain-type developing unitxe2x80x9d), the scraped toner image due to the mechanical factor can be reduced, whereby the very high quality image with less disorder can be produced.
However, the core rotating fountain-type developing unit has a problem that the image has jitter because two magnet rollers are rotated to cause vibration due to a magnetic field interference between the magnetic poles.
Also, in the two-component development, there is a problem that the magnetic force is temporally varied in strength in the developing area, and the carrier is likely to be attached onto the photosensitive body when the magnetic force is weak.
The temporal magnetic force changes in the developing area appear as xe2x80x9cfoggingxe2x80x9d in the one-component development.
The present invention has been achieved in the light of the above-mentioned problems. It is a first object of the invention to provide a developing unit and an electro-photographic apparatus that can print the high quality image by preventing jitter, splashing of carrier, and fogging from occurring due to vibration.
Further, in the previous method using two double rotation developing rolls, since a rotating magnetic field or a fixed magnetic field generated by the magnet roll of the first roll and a rotating magnetic field generated by the second roll pass each other in opposite directions at the most proximate point of two developing rolls, a magnetic interference between the first roll and the second roll occurs, or the magnetic poles attract or repel each other, so that the developing unit itself is vibrated around the rotation axis of the double rotation developing roll. Vibration of the developing unit causes the jitter, resulting in nonconformity that the striped unevenness appears on the image. With this constitution, two developing rollers must be fully separated from each other to avoid vibration due to interference of magnetic poles, resulting in a problem that the developing machine is larger in size.
In order to solve this problem, it is a second object of the present invention to provide an electro-photographic printer that can print a high definition image in a developing unit having a developing roll composed of a sleeve and a magnet roll that are both rotated by reducing vibration due to an interference between one magnetic pole of a rotating magnetic roll and the other magnetic pole.
In order to attain the first object, the invention provides a developing unit for developing an electrostatic latent image formed on a photosensitive body by supplying colored particles to the surface of the photosensitive body, which includes a plurality of developing rollers each having a core and a sleeve. The sleeve accommodates the core. The plurality of developing rollers includes a first developing roller and a second developing roller. The sleeve for the first developing roller is rotated to move in the same direction as a moving direction of the surface of the photosensitive body. The sleeve for the second developing roller is rotated to move in an opposite direction to the moving direction of the surface of the photosensitive body. The core for at least one of the plurality of developing rollers is driven in rotation.
In the developing unit as above constituted, one core is driven in rotation, thereby solving the problem that two cores are separately driven in rotation to give rise to vibration due to magnetic field interference between the magnetic poles.
Preferably, rotation of the core for one of the plurality of developing rollers follows rotation of the core for another one of the plurality of developing rollers.
In the developing unit as above constituted, one core is driven in rotation, and another core is freely rotated by following the rotation of the one core, whereby a balance point with the least force applied due to magnetic field interference is automatically generated to prevent the periodical vibration from occurring.
Preferably, the core for one of the plurality of developing rollers is fixed.
In the developing unit as above constituted, the sleeve is rotated with the core for one developing unit fixed to make the sleeve rotating method, whereby the fixed magnetic poles exist in the developing area opposed to the photosensitive body to exert a strong magnetic force at any time. Therefore, the carrier is attracted and not splashed, and the toner is not splashed to cause fogging on the background portion.
Also, the rotating sleeve cleans the carrier attached onto the photosensitive body or the toner of fogging on the background portion.
Preferably, the core is fixed via a damper.
In the developing unit as above constituted, since the core for the developing roller is fixed, there is the fear that the vibration occurs due to magnetic field interference of the core for the opposed developing roller, but since the core is fixed via the damper, an abrupt change in the torque is prevented by absorbing the rotation of the fixed core along with the rotation of opposed core.
The invention provides a developing unit for developing an electrostatic latent image formed on a photosensitive body by supplying colored particles to a surface of the photosensitive body, which includes first and second developing rollers each having a core and a sleeve. The sleeve accommodates the core. The core has magnetic poles. The cores of the first and second developing rollers have the same arrangement of the magnetic poles. The magnetic pole of the core of one of the first and second developing rollers is placed at an opposing position where first and second developing rollers mutually oppose, while the core of the other one of the first and second developing rollers is disposed such that a intermediate portion between the magnetic poles thereof is placed at the opposing position. The sleeve of the first developing roller is driven to rotate in the same direction as a moving direction of the surface of the photosensitive body. The sleeve of the second developing roller is driven to rotate in an opposite direction to the moving direction of the surface of the photosensitive body.
In the developing unit as above constituted, one sleeve is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the other sleeve is rotated to move in the opposite direction to the moving direction of the surface of photosensitive body, whereby the colored particles can be supplied onto the photosensitive body from two directions, and surely attached to the edge portion of the latent image formed on the surface of photosensitive body.
In the rotation of two cores having the same arrangement of magnetic poles, the magnetic pole of one core is placed between the magnetic poles of the other core, so that a constant magnetic force is applied on both the developing rollers to prevent vibration from occurring.
The invention provides an electro-photographic apparatus, which includes a photosensitive body, a charger for charging a surface of the photosensitive body, an exposing unit for exposing the surface of the photosensitive body charged by the charger to form an electrostatic latent image having different potential levels of at least two values on the surface of the photosensitive body, a developing unit for forming a visible image on the photosensitive body by supplying colored particles to the surface of the photosensitive body; and a transfer unit for transferring the visible image onto a recording medium. The developing unit includes a plurality of developing rollers each having a core and a sleeve. The sleeve accommodates the core. The plurality of developing rollers includes a first developing roller and a second developing roller. The sleeve for the first developing roller is rotated to move in the same direction as a moving direction of the surface of the photosensitive body. The sleeve for the second developing roller is rotated to move in an opposite direction to the moving direction of the surface of the photosensitive body. The core for at least one of the plurality of developing rollers is driven in rotation.
In the electro-photographic apparatus as above constituted, the sleeve for at least one developing roller is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the sleeve for at least one of the other developing rollers is rotated to move in the opposite direction to the moving direction of the surface of the photosensitive body, whereby colored particles are supplied onto the photosensitive body from two directions.
At this time, since only one core is driven in rotation, it is possible to solve the problem that two cores are separately driven in rotation as conventionally to give rise to vibration due to magnetic field interference between the magnetic poles.
Preferably, rotation of the core for one of the plurality of developing rollers follows rotation of the core for another one of the plurality of developing rollers.
In the electro-photographic apparatus as above constituted, one core is driven in rotation, and the other core is freely rotated by following the rotation of the one core, whereby a balance point with the least force applied due to magnetic field interference is automatically generated to prevent the periodical vibration from occurring. Preferably, the core for one of the plurality of developing rollers is fixed.
In the electro-photographic apparatus as above constituted, the sleeve is rotated with the core for one developing unit fixed to make the sleeve rotating method, whereby the fixed magnetic poles exist in the developing area opposed to the photosensitive body to exert a strong magnetic force at anytime. Therefore, the carrier is attracted and not splashed, and the toner is not splashed to cause fogging on the background portion.
Also, the rotating sleeve cleans the carrier attached onto the photosensitive body or the toner of fogging on the background portion.
Preferably, the fixed core is fixed via a damper.
In the electro-photographic apparatus as above constituted, since the core for the developing roller is fixed, there is the fear that the vibration occurs due to magnetic field interference of the core for the opposed developing roller, but since the core is fixed via the damper, an abrupt change in the torque is prevented by absorbing the rotation of the fixed core along with the rotation of opposed core.
The invention also provides an electro-photographic apparatus, which includes a photosensitive body, a charger for charging a surface of the photosensitive body, an exposing unit for exposing the surface of the photosensitive body charged by the charger to form an electrostatic latent image having different potential levels of at least two values on the surface of the photosensitive body, a developing unit for forming a visible image on the photosensitive body by supplying colored particles to the surface of the photosensitive body, and a transfer unit for transferring the visible image onto a recording medium. The developing unit includes first and second developing rollers each having a core and a sleeve. The sleeve accommodates the core. The core has magnetic poles. The cores of the first and second developing rollers have the same arrangement of the magnetic poles. The magnetic pole of the core of one of the first and second developing rollers is placed at an opposing position where first and second developing rollers mutually oppose, while the core of the other one of the first and second developing rollers is disposed such that an intermediate portion between the magnetic poles thereof is placed at the opposing position. The sleeve of the first developing roller is driven to rotate in the same direction as a moving direction of the surface of the photosensitive body. The sleeve of the second developing roller is driven to rotate in an opposite direction to the moving direction of the surface of the photosensitive body.
In the electro-photographic apparatus as above constituted, one sleeve is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the other sleeve is rotated to move in the opposite direction to the moving direction of the surface of photosensitive body, whereby the colored particles can be supplied onto the photosensitive body from two directions, and surely attached to the edge portion of the latent image formed on the surface of photosensitive body.
In the rotation of two cores having the same arrangement of magnetic poles, the magnetic pole of one core is placed between the magnetic poles of the other core, so that a constant magnetic force is applied on both the developing rollers to prevent vibration from occurring.